Such a method is known from a publication in SCANNING, Vol. 19, (1997) pages 553-563, entitled “A Robust Focusing and Astigmatism Correction Method for the Scanning Electron Microscope”. This article describes a method for focusing a beam of electrically-charged particles, whereby the electrically-charged particles are electrons. The particle-optical device with an imaging objective lens in which this method is implemented is a scanning electron microscope (SEM). In accordance with the method therein described, an image of a specimen in the particle-optical device is made at two differing settings of the imaging objective lens; subsequently, for each of the images, the spectral-energy content is determined in dependence upon the spatial frequency occurring in that image. This last-mentioned process is performed using a so-called Fast Fourier Transform (FFT).
Prior to making the two images, the nominal refractive power of the objective lens is first determined; that is to say the refractive power value for which the electron beam is approximately focused on the specimen. A deviation from this nominal setting is then applied, such that a setting of “over-focus” arises; additionally a deviation from the nominal setting is applied such that a setting of “under-focus” arises. The two images which are made at the two different settings of the imaging objective lens therefore consist of an “over-focus” image and an “under-focus” image. In order to automatically focus the electron beam, the total spectral energy content of both images is determined, as is the difference between the spectral energy content of the “over-focus” image and the “under-focus” image. The ratio R of this difference to the total spectral energy content gives a measure of the defocusing of the electron beam. If R is positive, then the “over-focus” image is sharper than the “under-focus” image and the focal length must therefore be shortened; if R is negative. the “under-focus” image is sharper than the “over-focus” image and the focal length must therefore be lengthened.
Along with this method for focusing the electron beam, a method is described for minimizing the astigmatism of the electron beam. In this last mentioned method, the spectral energy content of a number of sectors of the image is determined for each of the two images, and, on the basis of the difference between the respective spectral energy contents, a decision is made as regards in which direction the astigmatism must be increased or decreased in order to arrive at a beam which is virtually free of astigmatism.
Both the method for focusing the electron beam and the method for minimizing the astigmatism of the electron beam are described in particular in the aforementioned article on page 558 from equation (2) onwards to page 559 up to the paragraph entitled “Implementation”. It should be clear that, to automatically focus the electron beam, use is only made of the ratio between the spectral energy contents of both images, and not of the degree of astigmatism of the electron beams. In other words, with this known method it is possible to implement the focusing method without the presence of astigmatism in the beam which is to be focused.